The domestic feeder flight won't verify anything but will require your passport for the International Flight from SEA as it is on the same PNR.

Give yourself a little extra time at the gate in SEA. Usually boards from C2(X)* Only 76 seats on the Q400 but you will be surprised how many show up with nothing prepared. If your flight is operated on the E175 then you will likely board from C10(X) or C16(X).

The last time I flew SEA-YVR they were a little more organized than the time prior. There were four agents at the podium. They will take your passport, want to see your Molecular Covid test (not antigen) and confirm that you have an ArriveCAN receipt. (Shown in the ArriveCAN App after all your information has been submitted & verified.) They will print and put a sticker on your boarding pass once everything has been verified at the podium. Without the sticker, you will be denied boarding.

*C2(X) is on the left as you turn into the C concourse, it is a series of podiums lettered C2(B-M) (one gate on each side of the podium) so the X in parentheses will be one of those letters. C2(X) gates feed outside to a common outdoor ramp. At the bottom of the ramp, at ground level, you enter a covered corridor with numbered doors. There is overhead electronic signage indicating which door to enter for your flight. C10(A-F) and C16(A-F) are configured similarly.

Too many people were getting confused and getting on the wrong plane so they will rescan your boarding pass at the doorway.

The actual area on the apron/ramp/flight-line where the aircraft boards are numbered Q1, Q2, Q3.... so if you hear a pilot saying you will be arriving at gate Q6 you will be arriving at C2, door 1.

James